Bandpass and band-rejection filters have been widely used to control the flow of signals that propagate in electronic circuits. A bandpass filter is an electrical filter that allows a band of frequencies comprising a signal to pass through the circuit with minimal loss. A band-rejection filter, however, is an electrical filter that rejects or suppresses a band of frequencies.
In microwave and millimeter-wave receiver systems, undesired signals can often appear in the frequency band of interest. As a result, these unwanted signals can create interference problems within the receiver or they can saturate the amplifier of the receiver and "choke-off" the desired signals from the receiver system. To reduce the effect of these unwanted signals, those skilled in the art have incorporated tunable RF notch filters into the front-end of the receiver system. These notch filters essentially reject or suppress a narrowband of frequencies within the band of frequencies for which the receiver operates. Typically, such notch filters can be tuned to reject a narrow band or rejection band within the band of frequencies for which the receiver operates.
One such type of notch filter, a yttrium-iron-garnet (hereinafter YIG) filter is comprised of a plurality of magnetically tunable YIG spheres which basically act like tuned circuits. These YIG spheres, however, are non-planar bulky devices, and thus are not desirable for systems using integrated circuits.
A type of microwave band-rejection filter that is more useful in such planar applications is a tunable band-rejection slot-line filter. A band-rejection slot-line filter is basically composed of a plurality of slot-line resonators etched in the ground plane of a microstrip. The term "slot-line resonator" refers to the slots etched in the metallic ground plane formed on the dielectric substrate which separates a microstrip from an opposing ground plane. These slot-line resonators, when fabricated on a ferrite substrate, can be tuned to suppress or stop a narrow-band of frequencies within a frequency band by applying a variable magnetic bias to the ferrite substrate. The efficiency of the frequency suppression of such a slot-line resonator is directly dependant on the "Q" of the slot-line structure.
Heretofore, however, such planar tunable slot-line resonators have a relatively low "Q". It has been determined that slot-line filters of the type described above have a "Q" on the order of less than 100, and thus can only provide about 20 db of suppression of the unwanted signal per slot-line. As a result, for a given bandwidth, prior art slot-line resonators have a limited ability to suppress unwanted signals.